doc/go1.18: add docs on MapIter.Reset

From:

https://go-review.googlesource.com/c/go/+/321891

Updates #47694

Change-Id: I2cdd5d9f3da62a5690cd2ef921ed48957c602d64
Reviewed-on: https://go-review.googlesource.com/c/go/+/368360


Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>